<Background Art of First and Second Aspects of the Present Invention>
A lithium ion secondary battery as a kind of non-aqueous electrolyte secondary battery is a secondary battery where lithium ion in an electrolyte is responsible for electrical conduction, and a secondary battery using a lithium metal oxide for the positive electrode and a carbon material such as graphite for the negative electrode is predominating. The lithium secondary battery is characterized by having, among secondary batteries, a high energy density and is expanding its application range from small equipment such as cellular phone to large equipment such as electric car.
One of the challenges for the lithium ion secondary battery is to prevent reduction (deterioration) in the battery capacity resulting from repetition of charging and discharging (enhancement of cycle characteristics). The cause of reduction in the cycle characteristics is considered to be, for example, a change (deterioration) of the electrode active material, electrolyte, electrolytic solution, etc., or an increase in the electrode resistance due to separation between an electrode foil and an electrode active material, and among others, one greatest cause includes expansion/contraction of the active material itself. As one improvement method therefor, it has been proposed to enhance the cycle characteristics by adding a fibrous carbon material into an electrode (see Patent Document 1).
In addition, since the carbon material added cannot sufficiently utilize its characteristics when aggregated in the electrode, it has been proposed to enhance the cycle characteristics by using a fibrous carbon material improved in the dispersibility in an electrode (see Patent Document 2).
<Background Art of Third and Fourth Aspects of the Present Invention>
As described in the background art of the first and second aspects of the present invention, one of the challenges for the lithium ion secondary battery is to prevent reduction (deterioration) in the battery capacity resulting from repetition of charging and discharging (enhancement of cycle characteristics).
For example, Patent Document 1 has proposed a negative electrode for a lithium secondary battery, wherein the negative electrode for a lithium secondary battery contains a negative electrode active material capable of storing/releasing lithium, an electrically conductive carbon material and a binder, the negative electrode active material is a graphitic material using natural or artificial graphite in which the interplanar spacing d(002) of (002) plane of the graphite structure as measured by powder X-ray diffraction is from 0.335 to 0.337 nm, the electrically conductive carbon material is a vapor grown carbon fiber in which the average fiber diameter is from 1 to 200 nm and the fiber has a hollow structure inside and has a structure of graphene sheets being laminated in a direction perpendicular to the length direction of the fiber and in which the interplanar spacing d(002) of (002) plane of the graphite structure as measured by powder X-ray diffraction is from 0.336 to 0.345 nm, and the vapor grown carbon fiber is contained in an amount of 0.1 to 10 mass % relative to the entire negative electrode without forming an agglomerate of 10 μm or more. In addition, for example, Patent Document 3 has proposed a negative electrode active material ingredient for a lithium secondary battery, containing at least either one graphite material of flake graphite or spherical graphite and a fibrous carbon forming a secondary particle having an average particle diameter of 10 to 30 μm.